1. Field of the Invention
The present invention relates to an image measurement device, a method for image measurement, and a computer readable medium storing a program for image measurement. In more detail, the present invention relates to an image measurement device for measuring a size of a workpiece based on edge position of the workpiece image.
2. Description of the Related Art
Generally, the image measurement device is a device for measuring the size of the workpiece based on the edge position of the workpiece image (Japanese patent publication No. 2009-300124, No. 2009-300125, No. 2010-019667). As usual, the workpiece is placed on a movable stage which can be moved in X-axis, Y-axis and Z-axis directions. It is possible to focus the workpiece image by moving the movable stage in the Z-axis direction and adjust the position of the workpiece within a view of the image measurement device by moving the movable stage in the X-axis and Y-axis directions.
Since the workpiece image is strictly similar figure to the workpiece regardless of the position of the movable stage in the z-axis direction, it is possible to detect the actual size of the workpiece by measuring a distance or an angle of the workpiece image. For measuring the size of the workpiece, the edge of the workpiece image is extracted. The edge extraction is performed by detecting the edge point by analyzing a brightness change of the workpiece image and fitting the plurality of the detected edge points with geometrical elements such as line and circular arc, therein the edge indicating a boundary between the workpiece and background is determined.
A conventional image measurement device which compares the workpiece image with a previously captured master image and calculates an error indicating displacement amounts between the edge positions of the workpiece image and the corresponding master image is known. The conventional image measurement device compares the calculated error with predetermined tolerance and makes a quality check on the edge shape of the workpiece. Such a conventional image measurement device can calculate the size such as the distance or angle of the each plurality of the workpieces to be measured sequentially for quality management of the same kind of the workpiece to be manufactured through the same manufacturing process and display calculated average values and variance values of the measured size.